1. Field of the Invention
The present invention relates to a method and a means for allocating time slots in a time division duplex communication system. Particularly, the method and the means for allocating time slots according to the present invention can be implemented in communication units of a time division duplex communication system, e.g. a base station and/or a mobile station of a telecommunication system.
2. Description of Related Art
In a time division duplex communication system, examples of which are the GSM-standard for outdoor mobile communication or the DECT-standard for indoor mobile telephone communication, the receiving and the transmitting channels are separated on a time basis. E.g. in the DECT-standard, the information is transmitted in time frames of a length of 10 ms having 24 time slots. The first 12 time slots are used for the downlink connection, that is the transmission of data from a base station to a mobile station. The last 12 time slots are used for the uplink connection, that is for the transmission of data from a mobile station to the base station. In the GSM-standard, a combination of a frequency division duplex and a time division duplex is used. The information is transmitted in time frames having 8 time slots, whereby the uplink connection is provided in a lower frequency band and the downlink connection is provided in a higher frequency band. Each of the 8 time slots in each frame is assigned to a different mobile station. Thereby, the time slots assigned to a certain mobile station within a downlink time frame are separated from the time slots assigned to the same mobile station in the uplink time frame by two time slots. In other words, if e.g. the first time slot in a downlink time frame is assigned to a certain mobile station, the fourth time slot in an uplink time frame is assigned to the same mobile station. The time basis separation of the uplink and downlink time slots enables the construction of the mobile stations to be made more simple, since the reception and the transmission of data does not take place simultaneously.
Since in telecommunication systems as e.g. the GSM-system the data transfer rate is restricted, attempts have been made to increase the data transfer rate by allocating more than one time slot per frame to a certain mobile station. In JP 05153033-A such a TD digital mobile telecommunication system is disclosed, in which the same frequency for transmitting and receiving information is used. 1 to N time slots within one uplink time frame are allocated to one mobile station and 1 to N time slots within one downlink time frame are allocated to one mobile station depending on the information volume to be transferred between the mobile station and a base station. Each frame is allocated either to the uplink transfer of data or the downlink transfer of data. The uplink time frames cannot be used for a downlink transfer of data, so that a strong asymmetric transmission of information with a large difference between the amount of uplink data and the amount of downlink data is not possible.
In JP 07107546-A, a TDMA radio communication system is disclosed, in which the ratio between the number of uplink and downlink time slots within one time frame or one super frame consisting of several time frames is changed according to the total amount of traffic between a base station and mobile stations. In case of fast changing data transfer amounts, the switching point within each frame between the uplink time slots and the downlink time slots often changes position. Every change of such a switching point requires a reallocation of several time slots for the different connected mobile stations. This known system therefore requires a complicated circuitry.
The slot allocation method disclosed in EP 654916-A2 suffers from the same problems.
The object of the present invention is therefore to provide a method and means for allocating time slots in a time division duplex communication system, which allow a simple and efficient time slot allocation for varying transfer information amounts.
This object is achieved by a method for allocating time slots according to claim 1 and a means for allocating time slots according to claim 8. Advantageous features of the present invention are defined in the respective subclaims.
The present invention relates to a method for allocating time slots in a time division duplex communication system, in which the information is transmitted in predetermined time frames having a predetermined number of time slots. In a GSM-system, the number of time slots per time frame is 8. Each time frame comprises a fixed block of one receiving time slot and one transmitting time slot being adjacent to each other. For the case where the method according to the present invention is implemented in a communication unit as e.g. a mobile station, the receiving time slot is a downlink time slot and the transmitting time slot is an uplink time slot. The method for allocating time slots according to the present invention comprises the step of allocating at least the time slot adjacent to the receiving time slot as additional receiving time slot and at least the time slot adjacent to the transmitting time slot as additional transmitting time slot dependent on an amount of information to be transferred. Thus, starting from the fixed block consisting of the receiving and the transmitting time slot, the time slots for receiving and transmitting are extended, whereby additional receiving time slots are added on the side of the receiving time slot of the fixed block and additional transmitting time slots are added on the side of the transmitting time, slot of the fixed block. Thereby, the additional time slots can be added or additionally allocated crossing the border of two adjacent time frames. In other words, the additional time slots can be extended from one time frame into an adjacent time frame.
Thus, even if a big difference between the uplink data amount and the downlink data amount occurs, the method and the means according to the present invention provides an efficient and simple possibility to transfer the information to be transferred asymetrically. Since the position of the switching point between the receiving time slot and the transmitting time slot is fixed due to the fixed block position, the method according to the present invention allows a transfer data amount change of a certain mobile station without the need of a reallocation of time slots for other mobile stations. Thus, the present invention is particularly advantageous in a multiple access communication system, in which one time frame is assigned to several communication units, e.g. several mobile stations.
Advantageously, the number of additional receiving time slots and the number of additional transmitting time slots are independent from each other. This means, that data or information can be transferred asymmetrically between two communication units. The receiving and the transmitting time slot of the fixed block can be allocated to a common first communication unit, e.g. a mobile station, whereby the transmitting time slot precedes or is earlier than the receiving time slot. In other words, the transmitting time slot is positioned in front of the receiving time slot on the time axis, so that problems in view of the timing advance can be provided. The timing advance means, that the base station has to receive an uplink time slot at a correct timing. To meet this requirement, the transmission timing of the uplink time slot is adjusted e.g. by a mobile station taking the propagation delay into consideration. Of course, the propagation delay is more important in outdoor environments, in which communication units as e.g. mobile stations are sometimes moved with high speed or in which multipath effects occur. The adjustment of the transmission timing of the uplink time slot is called timing advance. Here, the method of the present invention is implemented in a mobile station and if the transmitting time slot is earlier than the receiving time slot, the transmission timing of the uplink time slot transmitted from the mobile station to the station is not necessary, since the timing advance does not play a role in this case.
The additional time slots can either be allocated to the same first communication unit as the fixed block, or, in case of a multiple access communication system, one time frame is assigned to several communication units and the additional time slots are allocated to communication units different from said first communication unit. Even in a multiple access communication system, the present invention provides an advantageous possibility for an asymetric data transfer.
The above-mentioned timing advance only becomes important, if all the time slots of a time frame are used for data transfer. Even in case that the transmitting time slot is preceding the receiving time slot, in one position of the time frame another switching point between a transmitting time slot and a receiving time slot occurs. In this switching point, a receiving time slot is preceding a transmitting time slot, so that, e.g. in a mobile station, the timing advance leads to a possible overlap of the earlier receiving time slot into the later transmitting time slot. In this case, a guard period can be provided in at least one of the adjacent time slots. In other words, a guard period can be provided either in the earlier receiving time slot or in the later transmitting time slot to avoid problems due to the timing advance. Advantageously, the guard period is only provided at the end of the receiving time slot.
According to claim 8, a means for allocating time slots in a time division duplex communication system is provided, in which the information is transmitted in predetermined time frames having a predetermined number of time slots. Each time frame comprises a fixed block of one receiving time slot and one transmitting time slot being adjacent to each other. Said means for allocating time slots allocates at least the time slot adjacent to the receiving time slot as additional receiving time slot and at least the time slot adjacent to the transmitting time slot as additional transmitting time slot dependent on an amount of information to be transferred. Said means for allocating time slots according to the present invention can e.g. be implemented in a communication unit of a telecommunication system, as a mobile station and/or a base station. All statements above made in reference to the method for allocating time slots according to the present invention are identically true for the means for allocating time slots according to the present invention.